Nowadays, display panels of non-emissive display devices such as liquid crystal display devices are not self-emissive, and thus backlight sources are necessary to be provided to the display panels for backlight illumination. The backlight sources primarily can be classified into cold cathode fluorescent lamps, hot cathode fluorescent lamps, LEDs and other electroluminescent elements. Specially, since the LEDs have the advantages of high color saturation, without mercury pollution, long operating life, low power consumption and adjustable color temperature, and therefore are increasingly used as the backlight sources of liquid crystal display devices.
Referring to FIGS. 1 and 2, FIG. 1 showing a schematic structural diagram of a conventional LED backlight module, and FIG. 2 showing statuses of driving currents for multiple LED strings during the LED backlight module of FIG. 1 performing a local dimming operation. As illustrated in FIG. 1, the LED backlight module 10 includes a LED driver 12, a power transformer circuit 14 and multiple LED strings e.g., CH1˜CH4 (i.e., driving channels). The LED strings CH1˜CH4 are independently controlled, and each of the LED strings CH1˜CH4 includes multiple LEDs 16 connected in series. Alternatively, each of the LED strings CH1˜CH4 may include single one LED 16 instead. The LED driver 12 includes a control circuit 121 and a current sink circuit 123. The control circuit 121 receives a digital input to set duty-on periods of driving currents flowing through the respective LED strings CH1˜CH4 in a frequency period and then provides the duty-on periods to the current sink circuit 123 for use. The current sink circuit 123 controls light emitting times of LED for the respective LED strings CH1˜CH4 in the frequency period according to the received duty-on periods. Moreover, the current sink circuit 123 is electrically coupled to receive a LED maximum current setting level Iset, and current values IFP of the driving currents for the respective LED strings CH1˜CH4 are set to be equal to Iset, for example 20 mA as shown in FIG. 2. The power transformer circuit 14 receives an input voltage Vin and then performs a voltage transforming operation to provide the LED driver 12 with an operating voltage and further provide the LED strings CH1˜CH4 with a power supply voltage Vout.